Over one third of our planet is frozen, and yet the icy worlds of the Arctic and Antarctic are as alien to most of us as the surface of another planet. They are places of superlatives, from ice caps that hold nearly 80% of our planet’s fresh water to frozen forests that encircle the entire globe. These are places that feed our imagination, places that seem to be borrowed from fairy tales. They are dominated and shaped by the ice, both by its coming and by its going. This is out planet’s last true wilderness, and one that is changing just as we are beginning to understand it.